Zinc oxide (henceforth referred to as “ZnO”) is widely used in various fields because it is an inexpensive, nontoxic material and it has various properties such as a semiconductor characteristic, an electrical conductivity and a piezoelectric property.
On the other hand, theories, technologies and devices of nanometer level have recently been under active study, and the development of ultrafine particle materials and ultrafine particle thin films has been demanded in many technical fields.
In particular, ultrafine particles of the ZnO are used for cosmetics, e.g., a sunscreen agent, and medical and pharmaceutical applications, e.g., a drug carrier, and ZnO is a direct transition type semiconductor having excellent luminous efficiency. Therefore, ZnO has been studied as a photocatalyst, in a UV shielding paint, and in emissive materials such as UV laser/LED, and its application to a gas sensor, an optical sensor, semiconductor conduction type control, an electrode material for dye-sensitized solar cells, and the like, has been studied actively.
Such ultrafine ZnO particles have conventionally been produced by various methods, such as a solid phase process, a gas phase process and a liquid phase process.
As a method for producing ultrafine ZnO particles by a solid phase process, known is a pulverization process in which large particles are mechanically pulverized to form fine particles.
However, although this pulverization process is a method suitable for producing a fine particle raw material at a low cost, impurities are easily incorporated into ZnO and it is difficult to obtain ZnO particles with a high purity because of the use of a pulverizing medium. Moreover, the pulverization process has a limit in the degree of particle size reduction because ZnO having a relatively large particle diameter is pulverized, and it is difficult to obtain ultrafine ZnO particles, that is, particles of nanometer level, and having a uniform particle diameter (as they have a wide particle size distribution).
As a method of producing an ultrafine ZnO particle thin film by a gas phase process, known are various methods such as a sputtering process, a molecular beam epitaxy process, a chemical vapor growth process and a laser ablation process.
However, these gas phase processes need an expensive equipment environment such as ultra-high vacuum, high voltage, high heat and a laser, and moreover, there is a need to control the environment strictly.
The liquid phase process can produce ultrafine ZnO particles in a comparatively simple environment compared with the gas phase processes. As a method for producing ultrafine ZnO particles by a liquid phase process, there have conventionally been known a hydrothermal synthesis process, a sol-gel process, a microemulsion process, and the like.
However, the hydrothermal synthesis process needs a large-scale apparatus because a raw material powder is dissolved by using water as a solvent under a high temperature and a high pressure in an autoclave.
Although the sol-gel process can provide ultrafine ZnO particles by hydrolyzing a Zn alkoxide, it is necessary to carry out a very slow hydrolysis reaction which requires several days.
Moreover, since dissolved raw materials in the hydrothermal synthesis process or the sol-gel process are reacted in a continuous phase, the particle size distribution tends to become wide, and agglomeration and sedimentation of particles easily occur. Therefore, it is difficult to obtain ultrafine ZnO particles with a uniform quality on the nanometer level. Furthermore, hydroxyl groups or hydrocarbons which are present in a solution may be incorporated into the ZnO particles, and impurities are easily incorporated into ZnO particles, so that the resulting ZnO thin film may become amorphous.
If the ultrafine particles have a size of 10 nm or less in such liquid phase processes, the particles easily agglomerate and, therefore, it is difficult to obtain ultrafine ZnO particles in a monodispersed state.
On the other hand, a microemulsion process is a process configured to obtain ultrafine particles by producing a water-in-oil (henceforth referred to as “W/O”) type microemulsion by mixing a hydrophobic solvent, a surfactant and water, then pouring raw materials into the microemulsion, and causing a hydrolysis reaction to take place. In this microemulsion process, it is conceivable that a high-purity ultrafine particle material having a relatively narrow particle size distribution can be obtained because the ultrafine particles are formed through a hydrolysis reaction carried out in water droplets surrounded by a surfactant.
In patent document 1, there is proposed ultrafine ZnO particles produced by adding a Zn alkoxide or a Zn alkoxyalkoxide to a W/O type microemulsion phase of a surfactant-water-nonpolar organic liquid type or a surfactant-water-alkanol-nonpolar organic liquid type, and performing a hydrolysis reaction.
In the patent document 1, nonylphenol ethoxylate (C9H19—C6H4—O(CH2CH2O)6H) (Terginol NP-6) is used as a surfactant, cyclohexane is used as a nonpolar organic liquid, and Zn di-n-butoxide is used as a Zn alkoxide. After ammonia water is solubilized in the surfactant and cyclohexane by adding it so that the amount of water is about 2 to 8 times the amount of the surfactant, Zn di-n-butoxide is added and stirred, and thereby ultrafine ZnO particles having an average particle diameter of 300 Å (30 nm) are obtained.
Moreover, patent document 2, which relates to mixed metal oxide ultrafine particles such as barium titanate (BaTiO3), proposes an ultrafine metal oxide particle dispersion solution produced by a hydrolysis reaction of a raw material in a microemulsion containing a dispersing medium, which is a hydrophobic liquid, water and a surfactant, wherein the raw material is composed of a mixed metal alkoxide solution hybridized by mixing a plurality of metal alkoxides in alcohol, and the amount of water contained in the microemulsion is 0.95 to 3 times the amount of water necessary for the hydrolysis of the raw material.
A water-in-oil type microemulsion solution is obtained in the patent document 2 using cyclohexane as a hydrophobic liquid, para-nonylphenol ethoxylate ((p-C9H19)—C6H4—O—(CH2CH2O)10CH2CH2OH) (Terginol NP-10) as a surfactant, and 1-octanol as a secondary surfactant. A Ba—Ti mixed alkoxide solution is supplied to the microemulsion solution so that the amount of water in the microemulsion solution is 0.95 to 3 times the amount of water necessary for the hydrolysis of the Ba—Ti mixed alkoxide, and thereby an ultrafine BaTiO3 particle dispersion in which BaTiO3 ultrafine particles having an average particle diameter of 10 nm or less are dispersed is obtained.
Patent document 1: JP 2-59425 A (claims, from line 1 to line 6 on the lower right-hand section in page 4, and FIG. 1)
Patent document 2: JP 2004-300013 A